Field of the Invention
The present invention relates to an illumination optical device, an exposure apparatus, and a method of manufacturing an article.
Description of the Related Art
In lithography processes included in processes of manufacturing semiconductor devices or the like, exposure apparatuses that transfer patterns formed in originals (for example, reticles) onto substrates (for example, wafers in which resist layers are formed on the surfaces) via projection optical systems or the like are used. The exposure apparatuses include illumination optical devices that illuminate the originals with light fluxes from light sources. Here, when illumination of the originals by the illumination optical devices is not uniform, transferring of patterns to resists is not sufficient, and thus the exposure apparatuses may not provide devices with high quality. Further, when the illumination optical devices are unable to illuminate the originals with high illuminance, throughputs of all of the exposure apparatuses are affected. Accordingly, the illumination optical devices are required to illuminate the originals with substantially uniform illuminance. Accordingly, in illumination optical devices of the related art, uniformity of illuminance is improved by disposing optical members such as reflection type optical integrators between light sources and planes of illumination. Japanese Patent Application No. 7-201730 discloses an illumination unit improving uniformity of illuminance of a plane of illumination by adopting an optical rod (glass rod) as a reflection optical integrator and disposing an emission end surface of the optical rod at a conjugated position with the surface of an original. In the illumination unit, an angle of a light flux incident on the optical rod is varied by driving an optical system provided on a front stage side of the optical integrator.
In general, in order to cause an illuminance distribution on the emission end surface of the optical rod to be uniform, the cross-sectional shape of the optical rod is required to be polygonal so that the number of internal reflections of the incident light is sufficiently large. That is, the cross-sectional area of the cross-sectional surface vertical to the optical axis of the optical rod is required to be small or the optical rod is required to be long in an optical axis direction. However, when the cross-sectional area of the optical rod is small, a ratio of light straying from the incident end surface of the optical rod at the time of deviation of the position of the light incident on the optical rod increases. This means that a loss of the amount of light from a light source easily increases, which is not preferable. On the other hand, when the length of the optical rod is long, a loss of the amount of light inside the optical rod (a loss dependent on glass material transmittance in the case of a glass rod or a loss dependent on reflection efficiency of a reflection surface in the case of a hollow rod) may increase and the size of the optical illumination optical device may increase, which is not preferable. In particular, in the technology disclosed in Japanese Patent Application No. 7-201730, an optical rod with a length of about 500 mm in the optical axis direction is used. Thus, when such a long optical rod is used, there is a concern of the illuminance of the plane of illumination deteriorating or the size of the entire illumination optical device increasing.